Method for making an electric contact material

ABSTRACT

Novel electrical contact materials are provided. The materials are comprised of three bonded layers including a top layer of palladium, an intermediate layer of a silver alloy and a nickelcopper alloy spring layer.

United States Patent Shida et al. 1 Mar. 14, 1972 [54] METHOD FOR MAKING AN ELECTRIC CONTACT MATERIAL [56] References Cited [72] Inventors: Sankichi Shida, Nara; Tsunehiko UNITED STATES PATENTS Osaka 2,303,497 12/1942 Reeve ..29/504 [73] Assignee: Matsushita Electric Industrial Co., Ltd., 2,691,816 10/1954 Siegel Kadoma, Osaka, Japan 3,091,026 5/1963 Hill et a1. [22] Filed: June 30, 1970 3,514,840 6/1970 Pitler ..29/471.1 X [21] A l. N 51,278 Primary Examiner-John F. Campbell Assistant Examiner-Richard Bernard Lazarus Atwmey-Wenderoth, Lind & Ponack [30] Foreign Application Priority Data July 2, 1969 Japan ..44/53639 1571 ABSTRACT Novel electrical contact materials are provided. The materials [52] US. Cl ..29/47l.7, 29/ 194, 29/ 199, are comprised f th ee bonded layers including a top layer of 29/475, 29/ palladium, an intermediate layer of a silver alloy and a nickel- [51] Int. Cl ..B23k 31/02 copper alloy Spring |ayer [58] Field of Search ..29/475, 504,194, 199, 471.1,

13 Claims, 10 Drawing Figures PMENTEUMAR 14 m2 3, 6A8 355 SHEET 1 [1F 2 FIG.|

FIG. 2A

FIGZB FIG. 2C

FIGZD FIGSD INVENTORS SANKICHI SHIDA TSUNEHIKD TODO-Ropq ATTORNEYS METHOD FOR MAKING AN ELECTRIC CONTACT MATERIAL This invention relates to a method for making an electric contact material and particularly said electric contact material is in a three layer bonded sheet including a palladium alloy top layer, a silver alloy intermediate layer and nickle-copper alloy spring layer.

The advanced industry has required increasingly a more reliable electric contact material. The reliable electric contact material must be provided with a high resistance to chemical corrosion such as sulfurization and mechanical wear as well as a low contact resistance and a high spring action.

There have been paid various efforts in obtaining the reliable electric contact at a cost as low as possible. However, the electric contacts available commercially at the present are not entirely satisfactory with these requirements.

An object of this invention is to provide a method for making an electric contact material characterized by low contact resistance and excellent mechanical properties such as high modulus of elasticity and high fatigue strength. Another object of this invention is to provide a method for making an electric contact material in a three layer bonded sheet including a palladium alloy top layer, a silver alloy intermediate layer and nickle-copper alloy spring layer.

These and other objects of this invention will be apparent upon consideration of the following detailed description taken together with accompanying drawings wherein:

FIG. 1 is a cross sectional view of a three layer bonded sheet according to the present invention,

FIGS. 2A through 2D are a schematic illustration of production steps of a three layer bonded sheet of FIG. 1,

FIGS. 3A through 3D are another schematic illustration of production steps of a three layer bonded sheet of FIG. 1,

FIG. 4 is variations of contact resistance with palladium content of palladium-silver alloy after sulfurization.

A method for making an electric contact material according to the present invention comprises the following steps:

I. A step for heating a combination of a palladium alloy sheet and a silver alloy sheet having a bonding layer inserted therebetween under pressure at a first bonding temperature of 720 to 850 C., whereby said bonding layer diffuses into both said palladium alloy sheet and said silver alloy sheet to form a two layer bonded sheet and rolling the cooled two layer bonded sheet.

2. A second step for heating a combination of said two layer bonded sheet and a nickel-copper alloy sheet having another bonding layer inserted therebetween under pressure at a second bonding temperature of 700 to 830 C., so as to form a three layer bonded sheet having a nickel-copper alloy spring layer bonded to said two layer bonded sheet.

3. A third step for cooling said three layer bonded sheet to room temperature and rolling the cooled three layer bonded sheet into an electric contact material having a desired thickness.

Before proceeding with the detailed description of the present invention, the construction of electric contact material contemplated by the invention will be explained with reference to FIG. I. Reference character designates, as a whole, an electric contact material consisting essentially of a three layer bonded sheet which has the following layers integrated together in the order of top below; a palladium alloy top layer 1, a silver alloy intermediate layer 2 and a nickelcopper alloy spring layer 3. These layers I, 2 and 3 are bonded in a method described in detail hereinafter. The palladium alloy top layer I is to protect the silver alloy intermediate layer 2 from the sulfurization and oxidation during storage and operation. The nickel-copper alloy spring layer 3 is to provide the electric contact material 10 with spring action. The silver alloy intermediate layer 2 has a low electric resistance and acts as an electric contact part. A composition and thickness of each of three layers 1, 2 and 3 will be explained hereinafter.

Referring to FIGS. 2A through 2D, a method for making an electric contact material according to the present invention will be explained. The method comprises a combination of following steps:

I. A step for heating a combination 20 of a palladium alloy sheet 11 and a silver alloy sheet 12 having a bonding layer I4 inserted therebetween under pressure at a first bonding tem perature of 720 to 850 C. An operable pressure range from 5 to 20 kg./cm. and can be applied by any suitable and available method during heating. For example, the combination 20 is penched by two thick stainless steel plates which are clamped strongly at the four corners by bolts. After heating for given time which depends upon the size of the combination 20, the combination 20 is converted into a two layer bonded sheet 30 consisting of a palladium alloy top layer I and silver alloy intermediate layer 4. The bonding layer M diffuses away through the palladium alloy sheet 11 and the silver alloy sheet 12 during the heating and disappears when cooled to room temperature. As a result the compositions of the palladium alloy top layer 1 and the silver alloy intermediate layer 4 are different from the original palladium alloy sheet 11 and the original silver alloy sheet 12, respectively due to the diffusion of bonding layer M.

The palladium alloy sheet 11 is in a composition consisting essentially of a main ingredient of palladium, a first additive ingredient selected from the group consisting of nickel, cobalt and copper and a second additive ingredient selected from the group consisting of silver and copper. The bonding layer I I consisting essentially of a member selected from the group consisting of a copper layer and a combination of a copper layer and an indium layer. The bonding layer M can be formed by any suitable and available methods such as vacuum deposition or electrochemical deposition of bonding material on either palladium alloy sheet I] or silver alloy sheet 12. Another method is to insert bonding material foil between the palladium alloy sheet II and silver alloy sheet 12.

2. A second step for heating a combination d0 of a two layer bonded sheet 30 and a nickel-copper alloy sheet I3 having another bonding layer I5 inserted there'between under pressure at a second bonding temperature of 700 to 830 C. An operable pressure range from 30 to 70 kgjcm. and can be applied in a way similar to that of a first step (I). Said another bonding layer I5 has a composition essentially the same as that of said bonding layer Id and can be formed in a manner similar to that of the bonding layer I4. After heating for given time which depends upon the size of the combination M, the combination 10 is converted into a three layer bonded sheet 50 consisting ofa palladium alloy top layer I, a silver alloy intermediate layer 2 and a nickel-copper alloy spring layer 3. The another bonding layer 15 diffuses away through the silver alloy layer 4 and the nickel-copper alloy sheet I3 during the heating and disappears when cooled to room temperature. As a result, the composition of the silver alloy intermediate layer 2 and the nickel-copper alloy spring layer 3 are different from the original silver alloy intermediate layer I and the original nickel-copper alloy sheet I3, respectively clue to the diffusion of another bonding layer IS.

A heating atmosphere on bonding step l) and (2) must be non-oxidizing atmosphere such as nitrogen gas, argon gas or vacuum for prevention of oxidation of electric contact material. It is necessary that the second boiling temperature is always lower than the first bonding temperature.

3. A third step for rolling the cooled three layer bonded sheet 50 into an electric contact material I0 having a desired thickness. The suitable annealing temperature of the three layer bonded sheet 50 during cold rolling is 620 to 670 C. for 1 hour. This method makes it possible to form a fine electric contact material characterized by the strong bonding strength between each two layers.

Operable composition for the silver alloy sheet I2 consists essentially of 60 to 97 wt. percent of silver and 3 to 40 wt. percent of copper. Copper, indium, lead, tin, zinc, etc. and their combinations are useful for bonding layer M. In view of the electric contact characteristics, copper and indium are preferable. When the silver alloy sheet I2 is in a composition of 95 to 97 wt. percent of silver and 3 to 5 wt. percent of copper, each of two bonding layers 14 and 15 is preferably composed of a copper layer in view of the solidus temperature of silver alloy sheet 12.

When the silver alloy sheet 12 is in a composition of 60 to 94 wt. percent of silver and 6 to 40 wt. percent of copper, each of two bonding layers 14 and 15 must be composed of a combination of a copper layer 14-1 or 15-1 and indium layer 14-2 or 15-2 in view of the eutectic temperature of silver alloy sheet 12 as shown in FIGS. 3A through 3D in which similar characters designate components similar to those of FIGS. 2A through 2D. It has been discovered according to the present invention that a higher bonding strength can be obtained by facing the copper layer 15-1 to the nickel-copper alloy sheet 13. A combination of a copper layer 14-1 or 15-1 and an indium layer 14-2 or 15-2 reacts with silver-copper alloy to form silver-copper-indium eutectic composition having a melting point lower than that of silver-copper alloy.

A thickness of the two bonding layers 14 and 15 less than 20 microns results in a low bonding strength. The bonding layer 14 and 15 thicker than 50 microns causes larger amounts of copper to diffuse to a surface of the palladium alloy sheet 11 during heating at the first bonding temperature. The diffused copper on the surface impairs the electric contact characteristics. The bonding layer 15 thicker than 50 microns fails to form a complete eutectic melt and remains a part of copper unmelted. This impairs the bonding strength. Operable thickness of the two bonding layers 14 and 15 must be 20 to 50 microns.

In the combination of copper layer 14-1 or 15-1 and indium layer 14-2 or 15-2, a thickness ratio of the copper layer to indium layer preferably ranges from 1:1 to 1:2. An indium layer thicker than the ratio 1:1 produces a large amount of electric melt at an interface between the palladium alloy sheet 1 l and the silver alloy sheet 12 or between the two layer bonded sheet 30 and the nickel-copper alloy sheet 13. The large amount of eutectic melt leaks away from the interface and prevents a formation of smooth interface. This also impairs the bonding strength.

A foresaid palladium alloy top layer 1 is to protect the silver alloy intermediate layer 2 from a chemical erosion such as sulfurization. An operable thickness of said palladium alloy top layer 1 is 0.5 to 5 microns. In view of the sulfurization, and mechanical wear it is necessary that the palladium alloy top layer 1 has 40 to 95 wt. percent of palladium included therein when the electric contact material 10 is finally achieved. As shown in FIG. 4, the sulfurization limit is 40 wt. percent of palladium for palladium-silver alloy in view of the contact resistance. The necessity can be satisfied by employing a palladium alloy sheet 11 in a composition listed in table 1.

Addition of l to 6 wt. percent of nickel or cobalt is effective in strengthening the palladium alloy top layer 1. Nickel or cobalt more than 6 wt. percent is apt to segregate and impair the ductility and workability of palladium alloy sheet 11. Palladium-nickel or palladium-cobalt alloy without silver and/or copper causes silver and/or copper to diffuse irregularly from the silver alloy sheet 12 and the bonding layer 14. The irregular diffusion results in a dappled surface of palladium alloy top layer 1. An addition of copper or silver of at least 2 wt. percent can prevent the irregular diffusion of silver and/or copper in the palladium alloy top layer 1. Upper limit of copper addition is 15 wt. percent in view of the electric contact characteristics. Upon limit of silver addition is 39 wt. percent in view of the sulfurization of palladium alloy top layer 1.

Both copper and silver addition to palladium without nickel or cobalt is also operable. In view of mechanical properties, electric contact characteristics and sulfurization, operable composition is shown by a sample No. 5 of table 1.

Silver alloys in a composition of table 2 are advantageous in view of mechanical properties and electric contact characteristics as intermediate layer. Copper less than 3 wt. percent does not provide the intermediate layer 2 with sufiicient mechanical properties. Copper above 40 wt. percent has no effect to increase the mechanical properties and impairs electric contact characteristics.

In view of the elasticity, fatigue strength and ductility, a composition listed in table 3 is useful for nickel-copper alloy sheet which forms finally into a spring layer. The carbon content in the nickel-copper alloy is important factor for the elasticity. Carbon content must be less than 0.08 wt. percent. Ductility and fatigue strength are damaged when carbon content is higher than 0.08 wt. percent.

The thickness of palladium alloy top layer 1 of rolled three layer bonded sheet 10 is 0.5 to 5 microns. The effect of palladium alloy top layer 1 against sulfide formation is not sufficient when thickness of palladium alloy top layer 1 is less than 0.5 microns. Above 5 microns, other convenient methods serve the purpose of making this type of electric contact material.

TABLE l-Composition of palladium alloy sheet Composition of silver alloy sheet Sample No. l 2

60 96.8 wt. '1: Ag 60 96.5 wt. I: Ag

3 39.95 wt. Cu 3 37 wt. 1; Cu 0.05 0.2 wt. 1: P 0.5 3 wt. k Ni TABLE 3 Composition of nickel-copper alloy sheet 63.0 70.0 wt. Ni less than 2.5 wt. Fe less than 1.25 wt. Mn less than 0.5 wt. Si less than 0.024 wt. S less than 0.08 wt. C remainder Cu EXAMPLE 1 A three layer electric contact material such as shown in FIG. 1 was made by following steps. Referring to FIG. 3, a palladium alloy sheet 11 was in a composition of wt. percent of palladium, 12 wt. percent of silver and 3 wt. percent of nickel and a silver alloy sheet 12 was in a composition of 85 wt. percent of silver, 13 wt. percent of copper and 2 wt. percent of nickel. Original thicknesses of the palladium alloy sheet 11 and the silver alloy sheet 12 were 0.3 and 4.2 mm. respectively. Both sheets were cleaned on their surfaces to remove gross contaminations by a usual manner. Then a copper layer 14-1 of 20 microns thick and an indium layer 14- 2 of 20 microns were electro-chemically deposited on the palladium alloy sheet 11 and silver alloy sheet 12 respectively. A combination 20 was penched under pressure of about 10 kgJcm. by two thick stainless steel plates which were clamped strongly at the four comers by bolts so that electrochemically deposited layers were faced closely to each other. The penched combination was held at 750 C. for 30 minutes in vacuum (10 mm. Hg). Thus, the combination 20 was converted into a two layer bonded sheet 30 of 1 mm. thick after three repetitions of a cycle of annealing at 550 C. for 30 minutes and cold-rolling of 40 percent reduction.

A nickel-copper alloy sheet 13 of 9 mm. thick was cleaned on its surface. A copper layer 15-1 of 20 microns thick was electro-chemically deposited on the nickel-copper alloy sheet 13 as shown in FIG. 30. An indium layer 15-2 of 20 microns thick was electro-chemically deposited on the silver alloy intermediate layer 4. The combination 40 was penched in a way similar to that of first step under pressure of about 50 kg./cm. and held at 700 C. for 30 minutes in vacuum mm. Hg).

Thus, three layer bonded sheet 50 was converted into an electric contact material 10 of 0.15 mm. thick after six repetitions of a cycle of annealing at 650 C. for 40 minutes and cold-rolling. The rolling process was followed by the annealing process every time when thickness of the three layer bonded sheet 50 was 5 mm., 2.4 mm., 1.2 mm., 0.6 mm., and 0.3 mm. Final reduction of thickness was 50 percent and the palladium alloy top layer 1 was in a thickness of about 1.5 micron by a microscopic examination. The palladium content of the surface of the palladium alloy top layer 1 was determined to be above 40 wt. percent by using microanalyzer. Other elements were mainly silver, copper and nickel. indium was detected as trace.

Table 4 shows the mechanical properties of so produced electric contact material. The electric contact material was subjected to a sulfurization test shown by table 4. After testing, the electric contact material had a contact resistance of 0.024 as shown in table 4. The sulfurization test was carried out by holding the electric contact material at 85 C. for 100 hours in air including 100 ppm. of H 8. The contact resistance was measured in the following manner. A gold electrode having a spherical surface at the end was brought into against a contact with the surface of electric contact material under pressure of 20 g. A direct current of 10 ma. was designed to flow from the GOLD electrode through the contact area to the electric contact material. The potential drop across the gold electrode and the electric contact material was measured by an electronic galvanometer and was calculated into a contact resistance.

EXAMPLE 2 Example 2 is substantially the same as example 1 and was made by the method described in example 1 except that a palladium alloy sheet 11 was in a composition of 95 wt. percent of palladium, 2 wt. percent of silver and 3 wt. percent of cobalt and that a silver alloy sheet 12 was in a composition of 60 wt. percent of silver, 37 wt. percent of copper and 3 wt. percent of nickel.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 3 Example 3 is substantially the same as example 1 and was made by the method described in example 1. Example 3 differs from example 1 in the following:

A palladium alloy sheet 11 was in a composition of 84 wt. percent of palladium, l5 wt. percent of copper and 1 wt. percent of nickel and silver alloy sheet 12 was in a composition of 93 wt. percent of silver, 6 wt. percent of copper and 1 wt. percent of nickel. Each of bonding layers 14 and 15 was a combination of copper and 30 microns thick and indium of 15 microns thick.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 4 Example 4 is substantially the same as example 1 and was made by the method described in example 1. Example 4 differs from example 1 in the following:

A palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 34 wt. percent of silver and 6 wt. percent of nickel and was in an original thickness of 1.35 mm. A silver alloy sheet 12 was in a composition of 60 wt. percent of silver, 39.95 wt. percent of copper and 0.05 wt. percent of phosphorous and was in an original thickness of 3.15 mm.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material and the palladium alloy top layer 1 was in a thickness of about 5 microns by a microscopic examination.

EXAMPLE 5 Example 5 is substantially the same as example I and was made by the method described in example I. Example 4 differs from example 1 in the following:

A palladium alloy sheet 11 was in a composition of 75 wt. percent of palladium, 15 wt. percent of copper and 6 wt. percent of cobalt and was in an original thickness of l.35 mm. A silver alloy sheet 12 was in a composition of wt. percent of silver, l3 wt. percent of copper and 2 wt. percent of nickel and was in an original thickness of 3.15 mm. Each of bonding layers 14 and 15 was a combination of copper of 25 microns thick and indium of 25 microns thick.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 6 Example 6 is substantially the same as example 1 and was made by the method described in example I. Example 6 differs from example l in the following:

A palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 25 wt. percent of silver and I5 wt. percent of copper and was in an original thickness of 1.2 mm. A silver alloy sheet 12 was in an original thickness of 3.3 mm. Each of bonding layers 14 and 15 was a combination of copper and 10 microns thick and indium of 10 microns thick.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 7 This example is substantially the same as example 1. A palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 39 wt. percent of silver and 1 wt. percent of cobalt, and was in an original thickness of 0.6 mm. A silver alloy sheet 12 was in a composition of 93 wt. percent of silver, 6 wt. percent of copper and 1 wt. percent of nickel and was in an original thickness of 8.4 mm.

After cleaning on their surfaces, a copper layer 14-1 of 20 microns thick and an indium layer 14-2 of 20 microns thick were electro-chemically deposited on the palladium alloy sheet 11 and silver alloy sheet 12 respectively. Then a combination 20 was bonded at 720 C. for 30 minutes in the same manner of example 1 and was converted into a two layer bonded sheet 30 of 1.2 mm. thick after two repetitions of a cycle of annealing at 550 C. for 20 minutes and cold-running of about 65 percent reduction.

A nickel-copper alloy sheet 13 of l0.8 mm. thick was cleaned on its surface. A copper layer 15-1 of 20 microns thick was electro-chemically deposited on the nickel-copper alloy sheet 13. An indium layer 15-2 of 20 microns thick was electro-chemically deposited on the silver alloy intermediate layer 41. A combination 40 was bonded at 700 C. for 30 minutes in the same manner of first step and was converted into an electric contact material 10 of 0.15 mm. thick after four repetitions of a cycle of annealing at 650 C. for 30 minutes and cold-rolling. The rolling process was followed by the annealing process every time when thickness of the three layer bonded sheet 50 was 9.6 mm., 2.4 mm. and 0.6 mm. Final reduction of thickness was 75 percent.

Table 4 shows the mechanical properties of so produced electric contact material. After sulfurization test carried out similarly to example 1, the electric contact material had a contact resistance of 0.038 as shown in table 4.

EXAMPLE 8 Example 8 is substantially the same as example 1 and was made by the method described in example 7 except that a palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 37 wt. percent of silver and 3 wt. percent of copper and that silver alloy sheet 12 was in a composition of 60 wt. percent of silver, 37 wt. percent of copper and 3 wt. 1 percent of copper and 3 wt. percent of nickel.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 9 2 Example 9 is substantially the same as example 1 and was made by the method described in example 7 except that a palladium alloy sheet 11 was in a composition of 84 wt. percent of palladium, wt. percent of copper and 1 wt. percent of EXAMPLE 10 Example 10 is substantially the same as example 1 and was 3 made by the method described in example 7. Example 3 differs from example 7 in the following:

A palladium alloy sheet 11 was in a composition of 95 wt. percent of palladium, 2 wt. percent of copper and 3 wt. percent of nickel and was in an original thickness of 0.2 mm. A

silver alloy sheet 12 was in an original thickness of 8.8 mm.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material and the palladium alloy top layer 1 was in a thickness of about 0.5 microns by a microscopic examination.

EXAMPLE 1] This example is substantially the same as example 1. Referring to FIG. 2, a palladium alloy sheet 11 was in a composition 5 of 95 wt. percent of palladium, 2 wt. percent of silver and 3 wt.. percent of nickel and was in an original thickness of 0.6 mm. A silver alloy sheet 12 was in a composition of 96.5 wt. percent of silver, 3 wt. percent of copper and 0.5 wt. percent of nickel and was in an original thickness of 8.4 mm. After clean- 5 ing on their surfaces, a copper layer 14 of 20 microns thick was electro-chemically deposited on the silver alloy sheet 12 and a combination 20 was penched in the same manner of example 1 so that the copper layer 14 and the palladium alloy sistance after sulfurization test of resultant electric contact material.

EXAMPLE 12 Example 12 is substantially the same as example 1 and was made by the method described in example 11 except that a palladium alloy sheet 11 was in a composition of 95 wt. percent of palladium, 2 wt. percent of silver and 3 wt. percent of copper and that a silver alloy sheet 12 was in a composition of 96,8 wt. percent of silver, 3 wt. percent of copper and 0.2 wt. percent of phosphorous.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 13 This example is substantially the same as example 1. Referring to FIG. 2, a palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 39 wt. percent of silver and 1 wt. percent of nickel and a silver alloy sheet 12 was in a composition of 96.5 wt. percent of silver, 3 wt. percent of copper and 0.5 wt. percent of nickel. Original thickness of the palladium alloy sheet 11 and the silver alloy sheet 12 were L2 and 3.3 mm. respectively. After both sheets were cleaned on their surfaces, a copper layer 14 of 30 microns thick was electrochemically deposited on the silver alloy sheet 12 and a combination 20 was penched under pressure of about 20 kgJcm. in the same manner of example 1 so that the copper layer 14 and the palladium alloy sheet 11 were faced closely to each other. The penched combination was held at 830 C. for 30 minutes in vacuum (10 mm. Hg). Thus the combination 20 was converted into a two layer bonded sheet 30 of 1 mm. thick in the same manner of example 1.

A nickel-copper alloy sheet 13 of 9 mm. thick was cleaned on its surface. A copper layer 15 of 30 microns thick was electro-chemically deposited on the nickel-copper alloy sheet 13. The combination 40 was penched under pressure of about 70 kg./cm. in the same manner of example I and held 830 C. for 30 minutes in vacuum 10 mm. Hg).

Thus three layer bonded sheet was converted into an electric contact material 10 of 0.15 mm. thick in the same manner of example 1 except that annealing condition was in a 5 temperature of 620 C. and was in a holding time of 1 hour.

sheet 11 were faced closely to each other. The penched com- A nickel-copper alloy sheet 13 of 10.8 mm. thick was cleaned on its surface. A copper layer 15 of 20 microns thick, was electro-chemically deposited on the nickel-copper alloyl sheet 13. The combination 40 was penched in the same manner of example 1 and held at 830 C. for 30 minutes in vacuum (10' mm. Hg). i Thus three layer bonded sheet 50 was converted into an electric contact material 10 of 0.15 mm. thick in the same manner of example 7.

Table 4 shows the mechanical properties and contact re- Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 14 Example 14 is substantially the same as example 1 and was made by the method described in example 13 except that a palladium alloy sheet 11 was in a composition of 60 wt. percent of palladium, 34 wt. percent of silver and 6 wt. percent of cobalt.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 15 Example 15 is substantially the same as example 1 and was made by the method described in example 13. Example 15 differs from example 13 in the following:

A palladium alloy sheet 11 was in a composition of 79 wt. percent of palladium, l5 wt. percent of copper and 6 wt. percent of nickel and a silver alloy sheet 12 was in a composition of 94 wt. percent of silver, 6.5 wt. percent of copper and 0.5 wt. percent of nickel. An annealing temperature of three layer bonded sheet 50 was 670 C.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

EXAMPLE 16 EXAMPLE 17 Example 17 is substantially the same as example 1 and was made by the method described in example 13. Example 17 differs from example 13 in the following:

A palladium alloy sheet 11 was in a composition of 95 wt. percent of palladium, 3 wt. percent of silver and 2 wt. percent of copper. Copper layers 14 and were in a thickness of microns. An annealing temperature of three layers bonded sheet 50 was 670 C.

Table 4 shows the mechanical properties and contact resistance after sulfurization test of resultant electric contact material.

TABLE 4 2. A method for making an electric contact material defined by claim 1, wherein said silver alloy sheet consists essentially of 95 to 97 wt. percent of silver and 3 to 5 wt. percent of copper and each of said bonding layer and said another bonding layer consists essentially of a copper layer in a thickness of 20 to 50 microns.

3. A method for making an electric contact material defined in claim 1, wherein said silver alloy sheet consists essentially of 60 to 94 wt. percent of silver and 6 to wt. percent of copper and each of said bonding layer and said another bonding layer consists of a combination of a copper layer and an indium layer.

41. A method for making an electric contact material defined in claim 3, said combination has a thickness of 20 to microns whereby a thickness ratio of an indium layer to said copper layer ranges from 1:1 to 1:2.

5. A method for making an electric contact material defined by claim 3, wherein said another bonding layer consists essentially of a combination of an indium layer and a copper layer which is adhered to said nickel-copper alloy sheet.

6. A method for making an electric contact material defined by claim 1, wherein said three layer bonded sheet has the palladium alloy top layer including 40 to 95 wt. percent of palladium.

7. A method for making an electric contact material defined by claim ll, wherein said original palladium alloy sheet is in a composition consisting essentially of l to 6 wt. percent of a Contact raslstance after Fatigue sulfurization Modulous limit for H28 100 uimp.,

Example of elasticity 10 cycles 85 C., 100 hours Number Construction (kg/mm?) (kg/mm (0) (85 Pd-12 Ag-3 Ni)-(85 Ag-13 Cir-2 N l)(nickel-coppcr alloy) 15, 700 37.0 0. 024

(95 Pd-2 Ag3 Co)-(60 Ag-37 (Du-3 N1)-(nickel-copper alloy) 15, 900 38. 0 O, 015

(84 Pd15 Cu-l Ni)(93 Ag-S Cu-1 Ni)-(nickel'coppcr alloy)-. 15,700 37.0 0. 025

(60 Pd-34 Ag-6 Ni)-(60 Ag39.95 Cu-O. 05 P)-(nickcl-copper alloy 15, 800 38. 0 0. 021

(79 Pd-15 Cit-6 Co)(85 Ag-13 011-2 Ni)(nickcl-coppcr alloy) 15, 700 38. 5 0. 014

(60 Fri-25 Ag-15 Cu)(85 Ag-13 Cu-2NO-(nickc1-copper alloy) 15, 800 39. 0 0. 018

(6O Pd-39 Ag1 Co)(93 Ag-fi Cu-l Ni)(nickcl-copper alloy) 200 38. 5 0.038

. (60 Fri-37 .Ag-3 Cu)(60 Ag-37 Cu-3 Ni)-(nickel-copper alloy) 16,300 39. 0 0. 029

9 (84 Pd-15 Cu-l C0)(94 Ag-5.5 Cu-Q.5 Ni)(nickel-eopper alloy) 16, 200 38. 5 0. 025 10 (95 Pd-2 Oil-3 N i)-(93 Ag-fi Cu-l N1)(nickel-copper alloy). 16,100 38.0 0.055 11. (95 Pd-2 Ag3 Ni)-(96.5 Ag-B Gil-0.5 Ni)(nickel-copper alloy) 16, 100 38. 0 0. 015 12 (95 Pd2 Ag-3 Cu)(96.8 Ag-B Cu0.2 P) -(nickcl-c0pper alloy). 16,100 38. 0 0. 013 13. (60 Fri-39 Ag-l Ni)(96.5 Ag-3 Gil-0.5 N1)(nickcl-copper alloy). 16, 200 39. 0 0. 022 (60 Pd-M Ag-G Co)(96.5 Ag-3 Oil-0.5 Ni)-(nickel-copper alloy)..." 16, 200 39. 0 0. 019

(79 Pd-15 Cu-6 N i)-( J4 Ag6.5 Ou-0.5 Ni)(nickel-copper alloy) 16, 000 38. 5 0. 014

16... (95 Pd-Z Cu-B Co)(96.5 Ag-3 Gil-0.5 N1)(ni ckel-copper alloy) 16, 000 38. 5 0.009 17 (95 Pd-3 Ag-2 CID-(96.5 .Ag-3 (Du-0.5 N i)(n1ckcl-copper alloy) 16, 000 38. 5 0. 008

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. A method for making an electric contact material comprising heating a combination of a palladium alloy sheet and a silver alloy sheet having a bonding layer inserted thcrebetween under pressure at a first bonding temperature of 720 to 850 C., whereby said bonding layer diffuses into both said palladium alloy sheet and said silver alloy sheet to form a two layer bonded sheet, said palladium alloy sheet being in a composition consisting essentially of a main ingredient of palladium, a first additive ingredient selected from the group consisting of nickel, cobalt and copper and a second additive ingredient selected from the group consisting of silver and copper and said bonding layer consisting essentially of a member selected from the group consisting of a copper layer and a combination of a copper layer and an indium layer;

cooling said two layer bonded sheet to room temperature and rolling;

heating a combination of the two layer bonded sheet and a nickel-copper alloy sheet having another bonding layer inserted therebetwcen under pressure at a second bonding temperature of 700 to 830 C. so as to form a three layer bonded sheet having a nickel-copper alloy spring layer bonded to said two layer bonded sheet;

cooling said three layer bonded sheet to room temperature;

and

rolling the cooled three layer bonded sheet.

metal selected from the group consisting of nickel and cobalt, 2 to 39 wt. percent of silver and 60 to wt. percent of palladium.

6. A method for making an electric contact material defined by claim 11, wherein said original palladium alloy sheet is in a composition consisting essentially of l to 6 wt. percent of a metal selected from the group consisting of nickel and cobalt, 2 to 15 wt. percent of copper and 79 to 95 wt. percent of palladium.

9. A method for making an electric contact metal defined by claim 1, wherein said original palladium alloy sheet is in a composition consisting essentially of 3 to 15 wt. percent of copper, 2 to 37 wt. percent copper, 2 to 37 wt. percent of silver and 60 to 95 wt. percent of palladium.

10. A method for making an electric contact material defined by claim 1, wherein said original silver alloy sheet is in a composition consisting essentially of 60 to 96.8 wt. percent of silver, 3 to 39.95 wt. percent of copper and 0.05 to 0.2 wt. percent of phosphorous.

11. A method for making an electric contact material defined by claim 1, wherein said original silver alloy sheet is in a composition consisting essentially of 60 to 96.5 wt. percent of silver, 3 to 37 wt. percent copper and 0.5 to 3 wt. percent of nickel.

12. A method for making an electric contact material defined by claim 1, wherein said original nickel-copper alloy sheet is in a composition consisting essentially of 63.0 to 70.0 wt. percent of nickel, less than'2.5 wt. percent of iron, less l l 12 than 1.25 wt. percent of manganese, less than 0.5 wt. percent defined by claim 1, wherein the rolled three layer bonded of silicon, less than 0.024 wt. percent of sulfur, less than 0.08 sheet has the palladium alloy top layer in a thickness of 0,5 to wt. percent of carbon and the remainder copper. 5 microns.

13. A method for making an electric contact material 

2. A method for making an electric contact material defined by claim 1, wherein said silver alloy sheet consists essentially of 95 to 97 wt. percent of silver and 3 to 5 wt. percent of copper and each of said bonding layer and said another bonding layer consists essentially of a coppeR layer in a thickness of 20 to 50 microns.
 3. A method for making an electric contact material defined claim 1, wherein said silver alloy sheet consists essentially of 60 to 94 wt. percent of silver and 6 to 40 wt. percent of copper and each of said bonding layer and said another bonding layer consists of a combination of a copper layer and an indium layer.
 4. A method for making an electric contact material defined in claim 3, said combination has a thickness of 20 to 50 microns whereby a thickness ratio of said indium layer to said copper layer ranges from 1:1 to 1:2.
 5. A method for making an electric contact material defined by claim 3, wherein said another bonding layer consists essentially of a combination of an indium layer and a copper layer which is adhered to said nickel-copper alloy sheet.
 6. A method for making an electric contact material defined by claim 1, wherein said three layer bonded sheet has the palladium alloy top layer including 40 to 95 wt. percent of palladium.
 7. A method for making an electric contact material defined by claim 1, wherein said original palladium alloy sheet is in a composition consisting essentially of 1 to 6 wt. percent of a metal selected from the group consisting of nickel and cobalt, 2 to 39 wt. percent of silver and 60 to 95 wt. percent of palladium.
 8. A method for making an electric contact material defined by claim 1, wherein said original palladium alloy sheet is in a composition consisting essentially of 1 to 6 wt. percent of a metal selected from the group consisting of nickel and cobalt, 2 to 15 wt. percent of copper and 79 to 95 wt. percent of palladium.
 9. A method for making an electric contact metal defined by claim 1, wherein said original palladium alloy sheet is in a composition consisting essentially of 3 to 15 wt. percent of copper, 2 to 37 wt. percent copper, 2 to 37 wt. percent of silver and 60 to 95 wt. percent of palladium.
 10. A method for making an electric contact material defined by claim 1, wherein said original silver alloy sheet is in a composition consisting essentially of 60 to 96.8 wt. percent of silver, 3 to 39.95 wt. percent of copper and 0.05 to 0.2 wt. percent of phosphorous.
 11. A method for making an electric contact material defined by claim 1, wherein said original silver alloy sheet is in a composition consisting essentially of 60 to 96.5 wt. percent of silver, 3 to 37 wt. percent copper and 0.5 to 3 wt. percent of nickel.
 12. A method for making an electric contact material defined by claim 1, wherein said original nickel-copper alloy sheet is in a composition consisting essentially of 63.0 to 70.0 wt. percent of nickel, less than 2.5 wt. percent of iron, less than 1.25 wt. percent of manganese, less than 0.5 wt. percent of silicon, less than 0.024 wt. percent of sulfur, less than 0.08 wt. percent of carbon and the remainder copper.
 13. A method for making an electric contact material defined by claim 1, wherein the rolled three layer bonded sheet has the palladium alloy top layer in a thickness of 0.5 to 5 microns. 